The concept of confined concrete has been widely accepted throughout the structural engineering field, such as the spiral, steel tube, or carbon fiber composite shell confined concrete, etc. In the present study, this concept has not only been used in the structural columns, but also in joints. In this connection system, the steel tube is interrupted at the floor, the concrete in the connection zone is confined by a stiffening ring with multiple lateral hoops, and the reinforced concrete beams are continuously arranged through the joint. The structural behavior of this new connection system in axial compression tests and reversed cyclic loading tests is presented in a companion paper. In the present study, the bearing strength of composite columns and the joint in axial compression is obtained based on the stress and strain analyses. The critical volume fraction of transverse stirrups of the composite column and the effective confining radius in the stiffening ring are proposed based on the thick cylinder model with the assumption of plane stress state. By using these solutions, it is favorable to obtain the stress distribution in confining concrete and the bearing strength of the confined concrete. For the reversed cyclic loading tests, the Clough hysteresis model is used to simulate the hysteresis loops of the specimens without considering the stiffness degradation in the unloading process. The results of the theoretical modeling are generally in good agreement with the experimental observations.